The invention relates to an NMR(=nuclear magnetic resonance) probe head with at least one capacitor that is motorized and tunable by means of a piezo-electric actuator in the RF(=radio frequency) resonant circuit of the NMR probe head, wherein the capacitor has a dielectric, which at least partially surrounds a cavity and which is connected to at least one electrode, at which a first and possibly a further electrical potential of the capacitor can be picked off, and wherein a capacitor piston with an electrically conductive surface is disposed in the cavity inside the dielectric and is displaced linearly by the action of the piezo-electric actuator by application of a sawtooth-waveform electric voltage to the piezo-electric actuator, the piezo-electric actuator being disposed outside the cavity as an extension of the axis of the capacitor piston.
Such an assembly is known from US 2008/0 117 560 A1(=Reference [10]).
NMR methods are used to analyze the composition of samples or to determine the structure of materials in samples. NMR spectroscopy is a powerful process of instrumental analysis. In this NMR process, the sample is exposed to a strong static magnetic field B0 in the z direction, to which orthogonal radio-frequency electromagnetic pulses are irradiated into the sample in the x or y direction. As a result, interaction with the nuclear spin of the sample material occurs. The development over time of these nuclear spins in the sample, in turn, generates radio-frequency electromagnetic fields, which are detected in the NMR apparatus. Information about the characteristics of the sample can be obtained from the detected RF fields. In particular, the position and intensity of the NMR lines provide information about the chemical bonds in the sample.
RF radiation is transmitted and received with so-called RF resonators. The RF resonators are disposed in close proximity to the sample, or the sample is disposed inside the RF resonators.
The electrical network of an NMR probe head usually consists of at least one electrical RF resonant circuit. There is one RF resonant circuit in the NMR probe head (see FIG. 2) for each atomic nucleus (e.g. 1H, 13C or 15N) that an NMR probe head can excite and detect for an NMR measurement of high quality, these RF resonant circuits must be finely tuned to each of the atomic nuclei to be measured in this way. As a rule, this is achieved with two continuously variable capacitors (tuning and matching) per RF resonant circuit. These are preferably rotary and/or sliding capacitors.
Besides continuously variable capacitors, there are also capacitors that can be adjusted in steps (switchable capacitors) with which the RF resonant circuits can be switched from one nuclear magnetic resonance to another. It is important that the electronic components (NMR coils, fixed and variable capacitors) of the RF resonant circuits be as close to each other as possible and that the electric wires extend over as short a distance as possible. Of course, they must be far enough apart to still guarantee the dielectric strength of the RF resonant circuits. This ensures that the electrical losses are kept to a minimum and the Q-value of the RF resonant circuits is as high as possible. Because in an NMR measurement the sample under examination is always moved into the magnetic center of the NMR magnet, not only the NMR coils but also the RF resonant circuits as a whole, including the variable capacitors, must be positioned as close as possible to the sample under examination in the NMR probe head. The problem lies in the accessibility of the variable capacitors, which usually come to lie several decimeters inside a magnet bore of a few centimeters in diameter and which must be tuned frequently (e.g. after each time the sample is changed and before the actual NMR measurement).
For the user of the NMR apparatus to be able to tune the relevant nuclear magnetic resonances of the RF resonant circuits via the variable capacitors, it must be easy to adjust the capacitors when they are installed in the NMR probe head. This can be achieved, for example, using adjustment rods, one end of which is attached to the variable electronic components and the other end of which is brought out to a location easily accessible by the user outside the NMR magnet. The adjustment rods are pushed and/or rotated depending on the type of variable capacitor. In addition to the adjustment rods, motors, cardan joints, flexible shafts [1, 2], gears and/or screw gears are frequently used. Motors with the relevant sensor technology permit automatic adjustment of the RF resonant circuits [3, 4].
These solutions have the following disadvantages:                1. Complex and fault-prone mechanisms (e.g. cardan joint and gears).        2. Hysteresis (compliance and play) in the mechanism. This impedes automatic adjustment, in particular.        3. Drift and slip-stick movements caused by thermal expansion. This results in unwanted detuning of the RF resonant circuits during the NMR experiment.        4. Because it has to be ensured that every single adjustment path is statically defined and free of stress, design and assembly thereof is correspondingly complex and demanding.        5. Electric motors [3] cannot be directly coupled to variable capacitors or located in the vicinity of the bore of the nuclear magnetic resonance magnet because the stray magnetic field of the nuclear magnetic resonance magnet is so high that the electric motors would be damaged (e.g. demagnetization of the permanent magnets) and the torque of the motor heavily reduced or even eliminated.        6. Moreover, for a cryo NMR probe head, a complex vacuum feed-through must be implemented for each adjustment path. Because of the stray magnetic field of the NMR magnet, lack of space, degassing problems and insufficient dissipation of heat in the vacuum, it is very difficult to locate conventional electric motors inside, or in the vacuum of, a cryogenic NMR probe head.        
In the vicinity of the variable capacitors (cylindrical volume with a diameter of approximately 40 mm), the (cryo) NMR probe head offers very little space for accommodating up to twelve electric motors (diameter approximately 10 mm). The electric motors are therefore mounted in the vicinity of the NMR probe head outside the NMR magnet or magnet bore and must be coupled to the variable capacitors, for example, by means of a coupling element (shafts, rods, gears, cardan joints).
The object of this invention is therefore to modify an NMR probe head of the type defined in the introduction using as simple technical measures as possible, so that the disadvantages listed above are largely eliminated without reducing the quality of the NMR measurements, wherein the NMR probe head is to be kept as compact as possible and the material costs and manufacturing complexity reduced.